Method of making nuclear magnetic resonance probe coil

ABSTRACT

A method of making a an NMR coil is provided. A coil is patterned of a film of a conductive material on a substrate. The coil mask is designed so that the resultant coil will have a lower resonant frequency than the desired frequency of the final coil. The coil is placed in an apparatus where it is exposed to increasing current, preferably within a magnetic field such as will be used during operation. The current is gradually increased and the coil observed for changes in its resonant frequency. When the coil is exposed to its operating current without further change in its resonant frequency, it is trimmed by removal of part of the capacitive element of the coil to the desired frequency.

STATEMENT OF GOVERNMENT INTEREST

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant No. 2R44 RR09757 awarded by the National Institutes of Health.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/409,506 by Richard S. Withers filed Mar. 23, 1995, now U.S.Pat. No. 5,585,723, and a continuation-in-part of U.S. patentapplication Ser. No. 08/313,624 by Richard S. Withers, Guo-Chun Liangand Marie Johansson filed Sep. 27, 1994, now abandoned, which is acontinuation-in-part of 891,591, U.S. Pat. No. 5,351,007 by Richard S.Withers and Guo-Chun Liang filed Jun. 1, 1992, each of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to RF probes for Nuclear Magnetic Resonancespectroscopy and microscopy. More particularly, it relates to resonantcoils for the transmission and reception of NMR signals. Even moreparticularly, it relates to superconductor coils on planar substrates.

2. Description of Related Art

In an NMR spectrometer probe, a sample is placed in a static magneticfield which causes atomic nuclei within the sample to align in thedirection of the field. Transmit and receive coils, which may becombined in a single coil or set of coils, are placed in the probepositioned close to the sample. The transmit coils apply an RF magneticfield orthogonal to the direction of the static magnetic field,perturbing the alignment of the nuclei. The transmit signal is thenturned off, and the resonant RF signal of the sample is detected by thereceiver coil.

The sensitivity of the spectrometer depends on a number of factors,including the strength of the static field, the closeness of thecoupling between the RF coils and the sample, and the resistance of theRF coil. Currently, all commercial NMR spectrometers use RF coils madeof a normal metal, such as copper, or a combination of normal metals.Much research has been devoted to the design of coils for maximumsensitivity. For example, to achieve close coupling, coils have beenmade in the form of solenoids, saddle coils and birdcage coils, all ofwhich have high filling factors. Similarly, researchers have suggestedcooling of RF coils to reduce their resistance. However, the sensitivityof conventional normal-metal coils is limited by their resistance to avalue less than that achievable with superconducting coils, even at lowtemperatures.

The use of superconductors in place of conventional normal metal for RFcoils in NMR spectrometers has previously been suggested. For example,Marek, U.S. Pat. No. 5,247,256, describes several RF receiver coilarrangements for NMR spectrometers using thin-film NbTi superconductingcoils. Marek's embodiments differ from the present invention in severalrespects. In particular, Marek's coils are nonplanar and use ohmiccontacts, both of which are easily realizable with NbTi.

The advantage to be obtained with high temperature superconductor("HTS") coils is significant. HTS coils would have very low resistanceand be operable in high magnetic fields at temperatures achievable withcurrently available refrigeration systems (above 20K). The qualityfactor, Q, of the coil is a useful measure of the coil's efficiency.Q=ωL/r, where ω is the resonant frequency, L is the inductance and r isthe resistance of the coil. Well-designed room temperature NMR coilsachieve loaded Qs of about 250. Because of the extremely low resistanceof HTS coils, coils with loaded Qs of 10,000 or more are possible.However, this advantage can only be realized if the other factorsnecessary for a superior NMR probe, reasonable filling factor and highRF and DC field homogeneity, are met. Thus, the ideal RF probe for NMRwould have a transmit/receive coil which would resonate at the desiredoperating frequency, produce a homogeneous RF field, not significantlydisturb the DC field, have a high filling factor, have a high Q, smallparasitic losses and produce a high RF magnetic field over the volume ofthe sample.

In addition to Marek, others have reported thin-film superconductor RFcoils for magnetic resonance applications. For example, Withers, U.S.Pat. No. 5,276,398 describes a thin-film HTS probe for magneticresonance imaging. It discloses a thin-film coil having inductors in aspiral of greater than one mm and capacitive elements extending from theinductors. Withers thus provides a thin film distributed capacitanceprobe coil. However, it does not address minimizing magnetic fielddisturbances by the coil, nor does it address maximizing the currentcarrying capacity of the coil.

Withers, et al., U.S. Ser. No. 08/313,624, which is incorporated hereinby reference, presents one type coil design suitable for NMRspectroscopy. It consists of a single loop with a single interdigitalcapacitor along one edge. Although its RF performance is adequate, ithas several deficiencies which the present invention corrects.Similarly, Black, U.S. Pat. No. 5,258,710, describes HTS thin-filmreceiver coils for NMR microscopy. Black discloses several embodiments,including split ting, solenoidal, saddle coils, birdcage coils and coilsdescribed as "Helmholti pairs." Black's embodiments are essentiallyconventional NMR coil designs and do not addressee uniquecharacteristics of high-temperature superconductor materials.

Superconductors are very attractive for use in these coils: They havevery low resistance at radio frequencies and, hence, produce littlenoise. Even so, to obtain high signal-to-noise ratio (SNR), the coilsmust be as close as possible to the sample. Unfortunately, this meansthat any magnetization of the coil material will affect the uniformityof the DC polarizing field (B₀) over the sample volume, producing adistortion of the spectral line shape and degradation of SNR. Becausesuperconductors are strongly diamagnetic line-shape distortions could besevere.

High temperature superconductors (HTS) are especially attractive for usein NMR coils because they may be operated at temperatures of 20 to 80K,permitting use of refrigeration units, rather than requiring the use ofliquid helium for cooling. However, thin-film HTS films have additionallimitations.

Thin-film HTS coils offer design and processing challenges not presentwith normal-metal coils. First, high-temperature superconductors areperovskite ceramics which require a well-oriented crystal structure foroptimum performance. Such orientation is extremely difficult to achieveon a nonplanar substrate. Generally, such coils are preferably depositedepitaxially on a planar substrate. This makes the achievement of a highfilling factor more challenging. It is also desirable for the coil to bedeposited in a single layer of superconducting film, without crossovers.Second, the coil must be able to handle relatively high currents whileproducing a uniform magnetic field and avoiding distortion of the B₀field of the magnet. Even when HTS films are deposited epitaxially on aplanar substrate, crystalline defects inevitably occur. This can lead toburn out of fine features of a coil exposed to high currents. Third, itis well known in the an that forming ohmic contacts between an HTS and anormal metal is difficult and generally leads to parasitic losses at thepoint of contact. To the extent a normal metal is used in the coil,resistive losses in the metal elements would lessen the advantagesgained from the use of the HTS. Thus, an ideal probe should avoidnormal-metal conductors. We are unaware of any prior an which takes intoconsideration the unique requirements for a superconducting NMRspectroscopy probe coil made from a high temperature superconductor.

SUMMARY OF THE INVENTION

An object of the invention is to provide a thin-film oxidesuperconductor NMR probe coil. It is a further object of the inventionto reduce the effect of the HTS coil on the uniformity of the polarizingfield throughout the sample volume. It is yet another object of theinvention to provide an NMR coil with increased RF current-carryingcapability of the coils. Another object of the invention is to providean NMR coil with reduced electric fields from the coil which fringe intothe sample volume and hence less noise. A further object of theinvention is to provide an NMR coil resistant to frequency shifts due toconductor burn out at operating currents.

These and other objects of the invention are met by providing an RF coilmade from a thin film of conductive material forming a plurality ofnested loops. Each of the loops is interrupted by a nonconductive areadefining a gap. Adjacent loops are nested so that the gap of any oneloop is not proximately disposed to the gap of an adjacent loop. Thecoil is of an essentially uniform width throughout. In a preferredembodiment, the gaps are symmetrically positioned to afford capacitiveelements with essentially equal capacitance. The segments of the loopsdefined by the gaps form fingers, with a finger being defined by asegment of the loop disposed between adjacent gaps (that is, 1/2 of aloop segment). The loops form an inductive element and adjacent fingersform a capacitive element. Preferably, the coil is formed from athin-film of a high temperature superconductor on a planar substrate,the coil being nearly symmetrical in both y and z, where z is thedirection of the polarizing field B₀ and y is the plane of the coilperpendicular to z. Preferably, the coil includes a plurality ofcapacitors in series.

In another embodiment of the invention, the fingers forming thecapacitors are each divided into fingerlets to reduce magnetization. Thefingerlets provide an advantage in any application in which an LCresonant structure is used and magnetization must be reduced. In yetanother embodiment, the capacitors are tapered in the direction of thecurrent to maintain a nearly constant current density throughout thecoil.

For NMR spectroscopy, these coils must both transmit and receive signalsof the appropriate frequency, typically in the range of 10 to 1000 MHzat currents of 10 amperes or more in the coil. This requires very highcurrent densities, as high as 10⁶ amperes per cm² or more, in the HTSthin film. Under such circumstances, we have frequently observed asudden and irreversible shift upward in the resonant frequency of thecoil, which typically makes the coil unusable. It is thought that thefrequency shifts because some small region of one of the capacitivefingers of the coil became resistive, resulting in a discharge of thestored energy of the coil and the destruction of the conductiveproperties of that small region. The capacitance of the severed fingeris removed from the circuit, so its resonant frequency rises.

We have devised a method to stress, or "burn in," the coils prior to afinal trimming process, so that those fingers which are likely to failwill do so prior to final trimming. An essential fast step is that thecoil design provide more capacitance than is needed, enough so that ifall defective fingers are removed, the coil resonant frequency is stillbelow the desired frequency of operation. Following burn-in, the coil istrimmed to the desired resonant frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Refering to the drawings in which like reference numbers refer to likeparts:

FIG. 1a depicts a coil according to U.S. Ser. No. 08/313,624;

FIG. 1b depicts the equivalent circuit of the coil of FIG. 1a;

FIG. 2a depicts a two-capacitor embodiment of the invention;

FIG. 2b depicts the equivalent circuit of the coil of FIG. 2a;

FIG. 2c depicts a variation of the coil of FIG. 2a, withoutinterconnects;

FIG. 3 depicts a four-capacitor embodiment of the invention;

FIG. 4 depicts a three-capacitor embodiment of the invention;

FIG. 5 depicts an embodiment of the invention having fingerlets;

FIGS. 6a, b and c depict an embodiment of the invention having taperedconductors;

FIG. 7 depicts a simplified two-capacitor coil;

FIG. 8 depicts the current and voltage distribution of the coil of FIG.7;

FIG. 9 depicts a simplified one-capacitor coil;

FIG. 10 depicts the current and voltage distribution of the coil of FIG.9;

FIG. 11 depicts a single capacitor embodiment of the invention;

FIG. 12 depicts a fixture for burn in of the coils prior to trimming;

FIG. 13 depicts the equivalent circuit of a burn in fixture with a coil;

FIG. 14 depicts the power coupled to the receive loop as a function offrequency for a coil in the burn in fixture.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1a depicts an NMR coil according to Withers et al. It consists of asingle loop 10 inductor of greater than one turn with a singleinterdigital capacitor 12 along one edge. The loop has a length and awidth, with squared corners 14. Other embodiments of Withers have acontinuous spiral structure or a spiral with crossovers. FIG. 1b depictsthe equivalent circuit of the coil of FIG. 1a. Although the RFperformance of the coil of FIG. 1a is adequate for some applications,this design has several limitations which the present invention seeks toavoid. First, it is asymmetric in the z direction, and to a lesserextent, in the y direction. It is preferable to avoid asymmetries inboth y and z, as they result in greater magnetic inhomogeneities overthe sample volume centered within the coil. Second, it has a singlecapacitor; consequently, the voltage ωLI developed along the loopinductance is present as a single electric dipole across nearly the fullcoil width, with an electric field reaching into the sample volume. Inan ideal coil a magnetic field, but no electric field other than thatinduced by the time-varying magnetic field, is coupled to the samplevolume. Third, current crowding occurs near the corners, limiting thecurrent carrying capacity of the coil as a whole. Fourth and mostimportantly, because the superconductor is in the form of a single lineexcept in the capacitor in response to the polarizing magnetic field, DCshielding currents flow in relatively large loops, which introduceslarge inhomogeneities in the polarizing field in the sample volume. Thecurrent invention provides an advantage over the coil of FIG. 1 byproviding structures that minimize magnetic disturbances while improvingother properties.

FIG. 2a depicts a first coil of the invention having greater symmetrythan the coil of FIG. 1a, achieved by the use of two capacitors inseries. FIG. 2a is not drawn to scale but, rather, is drawn so as tobetter illustrate the features of the invention. A superconducting filmforms two sets of elongated, interdigitated conductors 20, 22, formingloops of slightly less than one turn each with a gap 25 betweenterminals of the conductors. A set of conductors consists of one or moreconductors having their gaps aligned along a common radian of the coil.The gaps of the two sets of coils are symmetrically positioned withrespect to the coil's axis. Each loop is rounded at the corners 24 toavoid current crowding. The coil is of an essentially uniform width.Adjacent conductors between consecutive gaps around the perimeter of thecoil form a capacitor. As used herein, a section of conductor betweentwo gaps is a "finger." It may readily be seen that for a two-capacitorcoil, a loop consists of two fingers 30. Each loop of the coil has aslightly different perimeter from that of adjacent loops to accommodatea nested structure. The RF current flows around the loops. In FIG. 2a,each set of loops is joined at a node 26, 28. As shown in FIG. 2c anddiscussed below, the nodes are not necessary for a functional coil buttheir inclusion may improve coil yield. As shown in the equivalentcircuit, FIG. 2b, the interdigitated conductors form two capacitors inseries. While the coils of FIGS. 2a and 2c do not achieve perfect mirrorsymmetry about two planes, (that would require four symmetrically-placedcapacitors) they are significantly more symmetrical than the coil ofFIG. 1a.

For an NMR spectroscopy coil, the overall dimensions of the coil arechosen to maximize the filing factor for the sample volume. Suitableinside dimensions for a coil for use with a 5 mm sample tube are 1.7cm×1.2 cm. The number of fingers is then selected with reference to theprinciples discussed below and to achieve the desired resonant frequencyfor the probe assembly. In one embodiment having two capacitors, sevenloops were used, giving six gaps with 50 μm gaps between loops.

The principles disclosed in the coil of FIG. 2a may be extended to coilshaving N symmetrically positioned capacitors in series. In that case,each finger occupies 1/N of the perimeter of the coil, and eachinterdigitated conductor consists of one side of two adjacent capacitorswith a total length of slightly less than 2/N of the perimeter of thecoil. Conductors must be paired to complete loops with gaps separatingthe capacitive segments. Thus each loop extends over all N capacitors ofthe coil.

FIG. 3 depicts a coil having four capacitors symmetrically placed. Foursets of conductors 32 are symmetrically placed around the coil. Eachconductor has a length of 1/2 the perimeter of the coil minus the lengthof the gaps 25 between loops. Unlike the coil shown in FIG. 2a, in whichall fingers of a given side of each capacitor are shorted electricallyat "tie points" or connecting nodes at the top and bottom of the coil,no such nodes are present in the coil of FIG. 3. No voltage should bedeveloped between the several fingers on one electrical side of acapacitor, and thus these tie points may be eliminated withoutsignificantly affecting low-current performance. FIG. 4 depicts a coilhaving three sets of conductors 32 with three sets of gaps 25 placedsymmetrically around the coil forming three capacitors. It may readilybe seen that coils may be designed with any desired number ofcapacitors.

Use of two or more capacitors in series within the loop affects otherelements of the design. Because the effective capacitance of Ncapacitors of capacitance C in series is C/N, the gross capacitancerequired is N² times that required of a single-capacitor design. Inorder to achieve this level of capacitance in an interdigital design,finer lines are required. This has a corollary benefit in reducing thewidth of shielding-current loops and hence magnetic disturbances, but islimited by the ability of the film deposition and photolithographyprocesses to make defect-free coils with fine lines. To achieve therequired capacitance, the length of the capacitors is extended as muchas possible. Considerations related to selection of the length, width,inductance and capacitance of a coil are further discussed in Withers,U.S. Ser. No. 08/409,506.

In some cases, sufficiently small shielding currents are not obtainedwith single-line fingers of the width required for the desiredcapacitance. To further reduce the shielding currents, the individuallines of the capacitor fingers may be slit in the direction parallel tothat of the RF current flow so that a number "m" of "fingerlets" arecreated. This will not affect the RF performance (except for smalleffects in proportion to the amount of material removed), but reducesmagnetization effects by a factor of approximately m. FIG. 5 shows partof a design in which each finger 30 of the interdigital capacitors hasbeen slit into 4 or 6 fingerlets 44. The gaps between the fingerlets 42can be minimal (e.g., 10 μm), as little voltage is developed between thefingerlets, and a short circuit resulting from a photolithographicdefect will degrade B₀ uniformity only in proportion to the criticalcurrent of the bridging defect. The fingerlets may optionally be joinedby a common node 46.

FIG. 6a depicts a coil in which the interdigital conductors 50 aretapered along their length. The gap, or separation between conductors 52is kept constant over its entire length. FIG. 6b shows an enlarged tipof a single conductor. The tapered conductor 50 is truncated at its tip.FIG. 6c shows an enlarged view of a pair of adjacent tapered conductors50 with the uniform space 52 between conductors. Tapering the electrodesmakes optimal use of the current-carrying ability of the superconductingfilm. Because the current carried by each finger decreases linearlytoward its end as current is transferred to the fingers on the other(electrical) side of the capacitor, this approach maintains constantcurrent per unit of film width. By placing the conductor where it ismost needed, tapering also reduces the RF resistance of the coil, andhence its contribution to system noise. Similarly, the outermost andinnermost lines of the capacitors carry only half of the current of theinterior capacitors, and need only be one-half the width of the otherfingers. Similarly, because of their shorter length, the conductorsnearer the center of the coil carry less current than the outerconductors.

Some understanding of distributed-capacitor design can be gained by thefollowing analysis. Consider the design shown in FIG. 7. This is atwo-capacitor design that has been rendered in circular form to simplifyanalysis. The designed may be analyzed as follows:

Consider a current I_(o) (.o slashed.) flowing on the outer conductorand I_(i) (.o slashed.) flowing on the inner conductor. (All voltagesand currents are assumed to have an e^(j)ωt time dependence.) Thevoltages along the outer and inner conductors are V_(o) (.o slashed.)and V_(i) (.o slashed.), respectively. Conservation of current requiresthat I_(o) (.o slashed.)+I_(i) (.o slashed.)=I, independent of angle.There is an inductance per radian for current flow along eitherconductor of L (approximately the same for r_(o) ≈r_(i)). There is alsoa capacitance per radian between the conductors of C. We can then writethe following differential equations:

    dV.sub.o /d.o slashed.=dV.sub.i /d.o slashed.=-jωLI  (1)

and

    dI.sub.o /d.o slashed.=-dI.sub.i /d.o slashed.=-jωC)V.sub.o -V.sub.i)(2)

Equation (1) requires that the voltage difference between the inner andouter lines be independent of angle, i.e., V_(o) (.o slashed.)-V_(i) (.oslashed.)=ΔV, except at discontinuities. Equation (2) then requires thatthe inner and outer currents be simple linear functions of angle. Withthe boundary condition that the current be zero on the outer loop at .oslashed.=0 and zero on the inner loop at .o slashed.=π, we can derivethe current and voltage distributions shown in FIG. 8. The linearvariation in current along the length of each finger is the motivationfor linearly tapering the finger width, as shown in FIG. 6. The constantvoltage difference justifies a constant gap dimension (e.g., 50 μm). Thewidth of the gap is selected to withstand half of the peak transmitvoltage.

We can also derive the resonant frequency, f_(res) =(1/2π)(π² LC)⁻⁰.5.

In terms of the total inductance and capacitance of the coil, L_(T) =2πLand C_(T) =2πC, this is:

    f.sub.res =(1/2π)(L.sub.T C.sub.T /4).sup.-0.5.         (3)

Thus, the coil of FIG. 7 may be treated as a single-turn inductor with atuning capacitor which is one-fourth of the total distributedcapacitance. This coil design may be viewed as a planar, interdigitalversion of the coil design used for microscopy and described by Black,et al., "A probe for specimen magnetic resonance microscopy,"Investigative Radiology 27, 157 (1992) and Black et al., "Ahigh-temperature superconducting receiver for nuclear magnetic resonancemicroscopy," Science, vol. 259, p. 793, (1993). In fact, this analysiscorroborates the design guideline to treat the two capacitors on the twosides of the structure as having simply their lumped value.

The above analysis was derived in the context of FIG. 7 for interdigitalcapacitors having only one finger on each electrical side. Clearly, itcan be generalized to distributed capacitors of any sort (e.g.,interdigital, parallel plate) as long as they can be characterized bysome capacitance per unit length. For multifinger designs (FIG. 2a), wehave assumed that the capacitance per unit length is proportional to thenumber of neighboring edges on each side of the capacitor (e.g., thedesign of FIG. 2a has 6 edges in each capacitor).

For lower-frequency operation, such as is desired for deuterium locks orfor lower-field applications such as live-animal NMR microscopy, it maybe difficult to obtain sufficient capacitance using the above designapproach. FIG. 9 shows a modification which can effect a substantialreduction in frequency. This design uses, in effect, a single capacitordistributed around a single-turn inductor. The equations 1-3 apply tothis design as well as to that of FIG. 6; however, the boundaryconditions are very different. The current and voltage distributions inthis coil are shown in FIG. 10. Most importantly, the resonant frequencyis f_(res) =(1/2π)(L_(T) C_(T))⁻⁰.5, where L_(T) is the inductance ofthe (effectively) single-turn loop, and C_(T) is the total distributedcapacitance. It can be seen that the distributed nature of thecapacitance has no effect on the resonant frequency.

As shown in FIG. 11, the single capacitor design can be extended to morethan a single pair of fingers. The circular fingers 60 are tied to thecommon node (top) 62 in an alternating fashion from the left and right.The same analysis applies, but using the increased value of capacitanceper radian afforded by the multiplicity of fingers.

In the preferred embodiment, the superconducting films arehigh-temperature superconductors which possess low radio-frequencysurface resistance at temperatures of 77K or above. These materialsinclude YBaCuO, TlBaCaCuO, BiSrCaCuO, and related perovskite compounds.Deposition may be by sputtering, laser ablation, evaporation, orchemical vapor deposition. The substrate may be sapphire (Al₂ O₃),LaAlO₃, SrTiO₃, MgO, yttrium stabilized zirconium (YSZ), or othermaterials or combinations of these substrates with buffer layers.

The preferred embodiments of the invention, in particular thosecombining multiple capacitors with fingerlets, require conductive linesas narrow as 10 μm and an untuned resonant frequency of the coil verynear the desired resonant frequency. This presents two challenges.First, imperfections in the crystalline structure of the superconductorlines can result in burn out of conductors at the current loadingrequired for operation of the coil. Second, because the resonantfrequency of NMR spectroscopy coils must be tuned to a fractionalaccuracy which is much less than the reciprocal of the loaded qualityfactor (Q_(I)) of the coil, tuning is a significant challenge for NMRcoils made with high-temperature superconducting (HTS) thin films, whichhave loaded Qs in the range of 10,000. While real-time, reversibletuning using mechanically controlled means can be used to adjust theresonant frequency just before or during NMR measurements, it isdesirable to keep the mechanical tuning range to a minimum, as it comesat the expense of a lowered Q_(I) and increased vibrational sensitivity.

To address the problem of burn out of conductors and resultant shifts inthe resonant frequency of the coil, we have devised a method to stress,or "burn in," the coils prior to a final trimming process, so that thosefingers which are likely to fail will do so prior to final trimming. Anessential first step is that the coil design provide more capacitancethan is needed, enough so that if all defective fingers are removed, thecoil resonant frequency is still below the desired frequency ofoperation. The coil is typically designed with a resonant frequency 3 to5 per cent below the desired frequency of operation, allowing 6 to 10percent of the capacitance to be removed. For "burn in," the coil isfirst mounted in a simple fixture, shown in FIG. 12, that allows it tobe cooled in cold helium gas in a dewar. The fixture has two coaxialcables 80, 82 with small loops 84, 86, near the coil 90, terminating thecables. The positions of the two loops relative to the coil can bechanged by adjusting micrometers 92 mounted at the top of the fixture atroom temperature. FIG. 13 depicts the equivalent circuit of the coil inthe burn in apparatus, One cable 80 is connected to a source of RFpower. The loop 84 on this "transmit" cable is generally adjusted sothat the RF source is strongly coupled to the coil 90. The RF source isusually an amplifier 100 that is driven by a network analyzer 102. Theother loop 86 is connected to the network analyzer 102 so that theactual current in the coil can be monitored. This "receive" loop isweakly coupled to the coil, so that it does not significantly perturbthe power flow into the coil.

FIG. 14 is a plot of the power received from a coil in the burn inapparatus in a 7 Tesla field. The measurement and bum-in processproceeds as follows:

First, the RF source is set to a low power (typically 0.1 mW), and thetransmit and receive loops are adjusted so that the power coupled to thereceive loop is no more than 1% of the available power (20 dB or moreinsertion loss) and the reflected power from the coil at the resonantfrequency, f_(res), is minimized, i.e., the coil is matched to thesource, and all available power from the RF source is being dissipatedin the coil. This may be repeated at an even lower power to ensureoperation in a regime where coil resistance is independent of current.As the RF source frequency is swept through resonance, the power coupledto the receive loop traces the lowermost curve in FIG. 14.

Next, the coil matched quality factor, Q_(m), is calculated from thefull width Df of the resonant line at the 3-rib points from

    Q.sub.m =f.sub.res /Df

and the coil resistance R is calculated from

    R=(2πf.sub.res L)/(2 Q.sub.m)

where L is the coil inductance. The denominator is the unloaded Q, whichis twice the matched Q.

The rms current flowing in the coil at resonance under thesecircumstances is calculated from

    I=(P/R).sup.0.5

where P is the available power from the source. As shown in FIG. 14, forthe coil of the example, with -33 dBm (0.5 μW) available power, 14 mA isflowing through the coil.

The available power is then increased, and additional plots of powercoupled to the receive loop are made. Based on the proportionalrelationship I=k √P, with k established from the low-power measurement,the current at any point is readily computed. The available power isincreased until the observed current at resonance reaches the desiredbum-in level, which may be 5 to 50 amperes for typical NMR applications.This may require RF powers of 10W or more for typical coil matched Qs of10,000 and inductances of 30 nH. The RF sottree may be allowed to dwellat the resonant frequency of the coil for some extended period (seconds)in order to effect bum-in. Alternatively, the RF source may be pulsed tosimulate the NMR transmit operation. To most completely simulate theoperating environment of the coil, the bum-in process may be conductedwith the coil inside a magnet, as in FIG. 14.

If, at any time during the bum-in procedure, the resonant frequency ofthe coil is seen to shift, then the process is repeated from thebeginning, with the resonant frequency and the Q at low power beingremeasured. This is done until the coil can withstand the desired levelof etarent without undergoing a frequency shift.

After burn-in, coil tuning to the desired frequency is accomplished bytechniques such as photolithography, laser trimming, or ultrasonictrimming. The coil shown in FIG. 1a may be used as an example. Thedesired operational frequency of this particular coil is f_(op) =300MHz. The coil is designed such that its inductance L and capacitance Cgive a resonant frequency f_(des) =[2π(LC)⁰.5 ]⁻¹ of 295 MHz. The actualresonant frequency f₁ is measured using the RF network-analysistechniques discussed above, with weak inductive coupling to the probe.It is important that the probe be at the temperature at which it will beused for NMR measurements and in a similar electrical environment (e.g.,electrical shields are present as in the NMR probe to be used inmeasurement).

From the known dependence of resonant frequency on capacitance, thedesired fractional reduction in capacitance can be calculated to be, forsmall fractional corrections,

    ΔC/C=2(1-f.sub.1 /f.sub.op)

For the coil of FIG. 1a, capacitance is linear in the amount of edge oneither set of electrodes in the interdigital capacitor. Thus, an amountof finger which reduces the edge length by ΔL=L ΔC/C, where L is theinitial edge length, should be removed. This removal is accomplishedprecisely by covering the coil with photoresist and making one or moreexposures of the finger area to be removed with light through arectangular slit of variable length, width, and angular alignment on amicroscope. Measurement of the prescribed area to be removed is aided bysmall tic marks, i.e., a `ruler` provided on the pattern at regularintervals along the length of the finger area. The resist is developed,and the thin film is etched chemically or in an ion mill. It isimportant that the entire area be removed, not just cut off, as straycapacitance to the cut-off portion is found to be significant. Anaccuracy of 0.03% in one step has been achieved with this technique. Theprocess, which can be applied to the parallel-plate capacitors in othertypes of designs, does not degrade the electrical performance of thecoil.

A variant of the technique is to trim the coil capacitance in situ,i.e., while the resonant frequency is being measured. This approachrequires the use of a laser which is focused on the coil and appropriateoptics to view the coil during trimming in the low-temperaturemeasurement environment.

CONCLUSION

It will be appreciated by those skilled in the an that a number ofvariations are possible within the spirit and scope of the invention.For example, the invention is equally applicable to low temperaturesuperconductors, and to normal metal conductors at room or lowtemperature. While low temperature superconductor coils do not offer theadvantages of operating temperatures above 20K, they are capable ofachieving the extremely high Q. Normal metal coils do not achieve thehigh Qs of superconductor coils, but may benefit from application ofcertain aspects of the invention, and are at least minimally functional.It will also be appreciated that the coil design may be optimized forfilling factor, depending upon the size and shape of the sample, and maybe designed for any desired resonant frequency. Further, it will beappreciated that the invention is useful for NMR spectroscopy andmicroscopy and for magnetic resonance imaging.

While the foregoing disclosure contains many specificities, it should beunderstood that these are given by way of example only. The scope of theinvention should not be limited by the specific examples given above,but only by the appended claims and their legal equivalents.

What is claimed is:
 1. A method for making an NMR probe coilcomprising:(a) Depositing a film comprising an oxide superconductor on asubstrate; (b) Patterning the film to form a coil having an inductiveelement and a capacitive element wherein the coil has an expectedresonant frequency less than a desired resonant frequency of the coil;(c) Coupling RF power to the coil at or near the expected resonantfrequency; (d) Measuring a first observed resonant frequency of thecoil; (e) Increasing the RF power to the coil at or near its observedresonant frequency; (f) Observing the coil for a change in the resonantfrequency to a new observed resonant frequency; (g) Repeating theincreasing the RF power to the coil at or near the observed resonantfrequency until a desired power to the coil is reached; (h) Tuning thecoil by removing a portion of the capacitive element.
 2. The method ofclaim 1, wherein the substrate is selected from the group consisting ofsapphire, LaAlO₃, MgO SrTiO₃ and YSZ.
 3. The method of claim 1 whereinthe depositing is carried out by sputtering, laser ablation, evaporationor chemical vapor deposition.
 4. The method of claim 1 wherein theremoving is performed by providing a mask having tic marks at regularintervals.
 5. The method of claim 1 wherein the removing is carried outby photolithography, laser trimming or ultrasonic trimming.
 6. Themethod of claim 1 wherein the portion of the capacitive element removedis determined by the formula:ΔC/C=2(1-f₁ /f_(op)) where ΔC/C is thedesired proportional change in capacitance of the coil and f₁ /f_(op) isthe ratio of the observed resonant frequency of the coil to the desiredresonant frequence of the coil.
 7. The method of claim 1 furthercomprising the step of placing the coil in a magnetic field whileapplying input current.